Platelet membrane glycoprotein II (GPIII) is known to be the locus for the platelet alloantigen PLa1. This antigen is a gene product of one allele of a bi-allelic system having the observed distribution of: PLa1a1=0.69; P1a1a2=0.28; and PLa2a2=0.03. Exposure to the PLa1 antigen can induce alloimmunization in PLa2a2 individuals after platelet transfusions or during pregnancy in PLa2a2 mothers carrying PLa1a2 infants. This alloimmunization produces anti-PLa1 antibodies which can result in severe throbocytopenia in both post-transfusion purpura or in immune neonatal thrombocytopenia. Since a large fraction of the population is PLa1a1, they may in a similar manner, develop alloantibodies to the PLa2 antigen. This process may account for patients that become refractory to platelet transfusion during platelet support therapy and who show evidence of platelet specific antibodies unrelated to anti-HLA sensitization. Therefore, there is a significant clinical use for a system which can detect the presence of these specific anti-platelet antibodies and determine the phenotype of either recipients of platelet transfusions or expectant mothers. A new, simple and specific assay technique for detection of the anti-PLa1 or PLa2 antibodies in patient sera has been developed, and also, using the same technology, an assay for quantitation of these two antigens on platelet surfaces to determine the platelet PLa1/a2 phenotype was established. Both assays use labeled purified GPIII in a quantitative immunoprecipitation assay. The Phase I plan of this proposal will develop this assay into a biotin-avidin linked system having a format compatible with expansion to a commercial kit. Phase II of the project would involve development of this assay system to an inexpensive kit form and production for use in clinical evaluation.